  We report on theoretical studies of the recently discovered negative giant
magnetoresistance in ultraclean two-dimensional electron systems at low
temperatures. We adapt a transport model to a ultraclean scenario and calculate
the elastic scattering rate (electron-charged impurity) in a regime where the
Landau level width is much smaller than the cyclotron energy. We obtain that
for low magnetic fields the scattering rate and, as a consequence, the
longitudinal magnetoresistance dramatically drop because of the small density
of states between Landau levels. We also study the dependence of this striking
effect on temperature and an in-plane magnetic field.
