  The problem of a non-relativistic electron in the presence of a uniform
electromagnetic field and of one impurity, described by means of an
Aharonov-Bohm point-like vortex, is studied. The exact solution is found and
the quantum Hall's conductance turns out to be the same as in the impurity-free
case. This exactly solvable model seems to give indications, concerning the
possible microscopic mechanisms underlying the integer quantum Hall effect,
which sensibly deviate from some proposals available in the literature.
