  We have observed that the tunneling magnetoconductance between
two-dimensional (2D) electron gases formed at nominally identical InAs-AlSb
interfaces most often exhibits two sets of Shubnikov-de Haas oscillations with
almost the same frequency. This result is explained quantitatively with a model
of the conductance in which the 2D gases have different densities and can
tunnel between Landau levels with different quantum indices. When the epitaxial
growth conditions of the interfaces are optimized, the zero-bias
magnetoconductance shows a single set of oscillations, thus proving that the
asymmetry between the two electron gases can be eliminated.
