When you install Gnome, the latest Gnome (Gnome 3) will also update your Nautilus to the last version existing (now 3.6). So, if you want to downgrade your Nautilus from 3.6 to the old 3.4, you have to do the inversed process: remove the gnome3 ppa and update the system.
To remove a ppa, use the ppa-purge command, with sudo:
$ sudo ppa-purge ppa:gnome3-team/gnome3
$ sudo apt-get update
This is it, now your nautilus will be downgraded to 3.4.