  An unsaturated linear magnetoresistance (LMR) has attracted widely attention
because of potential applications and fundamental interest. By controlling
growth temperature, we realized a metal-to-insulator transition in Al2O3/SrTiO3
heterostructures. The LMR is observed in metallic samples with electron
mobility varying over three orders of magnitude. The observed LMR cannot be
explained by the guiding center diffusion model even in samples with very high
mobility. The slope of the observed LMR is proportional to Hall mobility, and
the crossover field, indicating a transition from quadratic (at low fields) to
linear (at high fields) field dependence, is proportional to the inverse Hall
mobility. This signifies that the classical model is valid to explain the
observed LMR. More importantly, we develop an analytical expression according
to the effective-medium theory that is equivalent to the classical model. And
the analytical expression describes the LMR data very well, confirming the
validity of the classical model.
