  We show that the the cyclotron resonance in a two-dimensional electron gas
has non-trivial properties if the correlation length of the disorder is larger
than the de Broglie wavelength: (a) the lineshape assumes three different forms
in strong, intermediate, and weak magnetic fields (b) at the transition from
the intermediate to the weak fields the linewidth suddenly collapses due to an
explosive growth in the fraction of electrons with a diffusive-type dynamics.
