  We perform a numerical investigation of the effect of the disorder associated
with randomly located impurities on shot noise in mesoscopic cavities. We show
that such a disorder becomes dominant in determining the noise behavior when
the amplitude of the potential fluctuations is comparable to the value of the
Fermi energy and for a large enough density of impurities. In contrast to
existing conjectures, random potential fluctuations are shown not to contribute
to achieving the chaotic regime whose signature is a Fano factor of 1/4, but,
rather, to the diffusive behavior typical of disordered conductors. In
particular, the 1/4 suppression factor expected for a symmetric cavity can be
achieved only in high-quality material, with a very low density of impurities.
As the disorder strength is increased, a relatively rapid transition of the
suppression factor from 1/4 to values typical of diffusive or quasi-diffusive
transport is observed. Finally, on the basis of a comparison between a
hard-wall and a realistic model of the cavity, we conclude that the specific
details of the confinement potential have a minor influence on noise.
