QUANTUM INTERFERENCE AND LANDAU-LEVEL BROADENING IN NARROW GAAS ALGAAS CHANNELS

High 2D electron mobility narrow Hall bars, 0.8-mu-m wide by 30-mu-m long, were fabricated in which, after photoexcitation, the scattering mean free path was up to 4.5-mu-m long at 1.3 K. The low field negative magnetoresistance arises from quantum interference (weak localization) with the flux cancellation mechanism [1] absent, even though the mean free path is much greater than the width. The Landau levels are significantly broadened, implying that a flat-bottomed model potential is inappropriate in this particular case. It is suggested that the potential fluctuations giving rise to the broadening are responsible for the absence of the flux cancellation, and when small angle scattering contributes to the elastic mean free path. The value of this length compared with the width is no longer a sufficient criterion for the determination of the nature of the magnetoresistance.