  An experimental analysis for two parallel conducting layers determines the
full resistivity tensor of the parallel layer, at magnetic fields where the
other layer is in the quantum Hall regime. In heterostructures which exhibit
parallel conduction in the modulation-doped layer, this analysis quantitatively
determines the charge density in the doping layer and can be used to estimate
the mobility. To illustrate one application, experimental data show magnetic
freeze-out of parallel conduction in a modulation doped heterojunction. As
another example, the carrier density of a minimally populated second subband in
a two-subband quantum well is determined. A simple formula is derived that can
estimate the carrier density in a highly resistive parallel layer from a single
Hall measurement of the total system.
