  The low-field quantum Hall effect is investigated on a two-dimensional
electron system in an AlGaAs/GaAs heterostructure. Magneto-oscillations
following the semiclassical Shubnikov-de Haas formula are observed even when
the emergence of the mobility gap shows the importance of quantum localization
effects. Moreover, the Lifshitz-Kosevich formula can survive as the oscillating
amplitude becomes large enough for the deviation to the Dingle factor. The
crossover from the semiclassical transport to the description of quantum
diffusion is discussed. From our study, the difference between the mobility and
cyclotron gaps indicates that some electron states away from the Landau-band
tails can be responsible for the semiclassical behaviors under low-field Landau
quantization.
