  We study the spin dynamics in a high-mobility two-dimensional electron gas
confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of
the spin relaxation is found: as the magnetic field becomes stronger, the spin
relaxation time first increases quadratically but then changes to a linear
dependence, before it eventually becomes oscillatory, whereby the longitudinal
and transverse times reach maximal values at even and odd filling Landau level
factors, respectively. We show that the suppression of spin relaxation is due
to the effect of electron gyration on the spin-orbit field, while the
oscillations correspond to oscillations of the density of states appearing at
low temperatures and high magnetic fields. The transition from quadratic to
linear dependence can be related to a transition from classical to Bohm
diffusion and reflects an anomalous behavior of the two-dimensional electron
gas analogous to that observed in magnetized plasmas.
