  By means of degenerate four-wave mixing (FWM), we investigate the quantum
coherence of electron-hole pairs in the presence of a two-dimensional electron
gas in modulation-doped GaAs-AlGaAs quantum wells in the quantum Hall effect
regime. With increasing magnetic field, we observe a crossover from Markovian
to non-Markovian behavior, as well as large jumps in the decay time of the FWM
signal at even Landau level filling factors. The main observations can be
qualitatively reproduced by a model which takes into account scattering by the
collective excitations of the two-dimensional electron gas.
