Reference for Bava Batra 23:5
אמר רב ענן אמר שמואל מבואות המפולשין לרה"ר ובקשו בני מבואות להעמיד להן דלתות בני רה"ר מעכבין עליהן
R. 'Anan said in the name of Samuel: If the residents of alleyways which open out on to the public thoroughfare desire to set up doors at the entrance, the public [who use the thoroughfares] can prevent them. It was thought that this right extended only to a distance of four cubits [from the public thoroughfare], in accordance with what R. Zera said in the name of R. Nahman, that the four cubits [in the alley-way] adjoining the public thoroughfare are on the same footing as the public thoroughfare. This, however, is not the case. For R. Nahman's rule applies only to the matter of uncleanness,<span class="x" onmousemove="('comment',' If there is a suspicion of uncleanness in the four cubits up the alley-way it is treated as if it occurred in a public place and is deemed clean. Toh. IV, II. ');"><sup>4</sup></span>