Dear Rabbi Kornfeld:
Rav Sheshet states (Eruvin 14a) that a hanging wall that does not reach within 3 Tefachim to the ground is not a valid Mechitza (i.e. we do not apply the rule of Lavud) because it is a "Mechitza Sh'Hagediyim Bokeem Bah" (a wall that goats can pass through). Rav Nisim Gaon points out that the source of this idea, BKiyat Gediyim, is the Tosefta Kelaim which is quoted later on in our Gemora (16a).
This concept however needs clarification. Why should goats invalidate a wall. The rules of Hotza'ah from the Midbar. We should therefore only consider human traffic and not animal traffic, in the same way only people are counted for the 600,000 constituting a Reshut Harabim - i.e. animals don't count. So why is goat traffic considered significant enough to invalidate a wall.
It is true that human traffic invalidates the walls of Pasei Beerot according to Rebbe Yehuda (22a) , as the gemora explains (20a) "Ati Rabim u'Mevatel Mechitzah". (The idea of Mechitza Nidreset is also dealt with later on (89b)). However the Rabanan (on 22a) disagree and hold "Lo Ati Rabim u'Mevatel Mechitzah". Furthermore, Rebbe Yehuda, who invalidates the wall, does so only when there is more than just the potential for traffic, but there is actual traffic. By Gediyim on the other hand we are dealing with just a potential for traffic. So why is potential goat traffic considered significant enough to invalidate a wall.
(a) Good question. The answer appears to be that the laws of Reshus ha'Rabim are learned from the Midbar, but not the laws of Reshus ha'Yachid. The laws of Reshus ha'Yachid are Halachos le'Moshe mi'Sinai (Eruvin 4b), and among them is the principle of Beki'is Gedi'im. The logic behind this Halachah is that anything which does not serve to separate one area from another and to stop things from entering it, is not considered to serve as a Mechitzah that demarcates a separate domain.
(b) Good point. Certainly, the presence of human traffic should, and will, invalidate a Mechitzah just like Beki'as Gedi'im. However, Beki'as Gedi'im can only invalidate that Mechitzah (or a part of Mechitzah) under which the goats walk, and not the other Mechitzos next to it. This is not so with regard to Rebbi Yehudah's opinion that "Asi Rabim u'Mevatel Mechitzah," whereby the human traffic invalidates even the other Mechitzos that surround the place where the traffic enters. This is the point of the Machlokes between the Rabanan and Rebbi Yehudah; the Rabanan maintain that the presence of human traffic does not invalidate the Mechitzos around that area since they are kosher Mechitzos. (The place where the people enter does not have to have a Mechitzah of itself, in the case under discussion, since it is an entranceway.)