Magnetoresistance in a GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructure with double subband occupancy

A temperature-independent positive weak-field magnetoresistance is observed in a high-mobility GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructure with two occupied two-dimensional subbands. Persistent photoconductivity is used to study this effect for various electron-gas densities. The magnetoresistance is interpreted in terms of a simple classical model for two independent electron gases characterized by their respective number densities and mobilities. This analysis allows a direct determination of the mobility of the second subband.