At low magnetic fields the experime

At low magnetic fields the experimental oscillations are damped more rapidly than the theory would predict, and, moreover, an unexplained positive magnetoresistance is observed around zero field in ρ⊥ (but not in ρ||). Part of this disagreement may be due to nonuniformities in the potential grating, which become especially important at low fields when the cyclotron orbit overlaps many modulation periods. At high magnetic fields B >∼0.4T the experimental data show the onset of Shubnikov-De Haas oscillations, which are a consequence of oscillations in the scattering time τ due to Landau level quantization (cf. Section I.D.3). This effect is neglected in the semiclassical analysis, which assumes a constant scattering time.