  The crossover from the semiclassical transport to quantum Hall effect is
studied by examining a two-dimensional electron system in an AlGaAs/GaAs
heterostructure. By probing the magneto-oscillations, it is shown that the
semiclassical Shubnikov-de Haas (SdH) formulation can be valid even when the
minima of the longitudinal resistivity approach zero. The extension of the
applicable range of the SdH theory could be due to the damping effects
resulting from disorder and temperature. Moreover, we observed plateau-plateau
transition like behavior with such an extension. From our study, it is
important to include the positive magnetoresistance to refine the SdH theory.
