Effect of a parallel magnetic field on tunneling in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterostructures

The tunneling of electrons between a quantum well and $n^{+}$ electrode is observed for various well densities and magnetic fields aligned parallel to the interfaces. We find that the maximum in the tunneling conductance shifts to higher well densities as the field is increased. Considerations of energy and transverse-momentum conservation in the presence of a parallel field predict a threshold in the tunneling conductance, in agreement with our data.