setting default window manager in 12.04

installed kubuntu 12.04 on a new box. later installed xfce4 as it is used as the desktop for some vncserver sessions. however, the install made xfce the default window manager. ie: new users get xfce as the default. as ubuntu does not use /etc/sysconfig, where is the default manager setting?

/etc/X11/default-display-manager is defined as kdm.

thanks.