  We investigate the behavior of the shot-noise power through quantum
mechanical cavities in the semiclassical limit of small electronic wavelength.
In the absence of impurity scattering, the Fano factor $F$, giving the noise to
current ratio, was previously found to disappear as more and more classical,
hence deterministic and noiseless transmission channels open up. We investigate
the behavior of $F$ as diffractive impurities are added inside the cavity. We
find that $F$ recovers its universal value provided (i) impurities cover the
full cavity so that only a set of zero measure of classical trajectories may
avoid them, and (ii) the impurity scattering rate exceeds the inverse dwell
time through the cavity. If condition (i) is not satisfied, $F$ saturates below
its universal value, even in the limit of strong scattering. Our results
corroborate the validity of the two-phase fluid model according to which the
electronic flow splits into two well separated components, a classical
deterministic fluid and a stochastic quantum-mechanical fluid. Only the latter
carries shot-noise.
