We have observed quantum interference in the electronic transport in a T-shaped Al 0.3 Ga 0.7 As/GaAs heterostructure. The geometry is defined by four independent Schottky gates on top of the layer system. By changing the split-gate voltages, the dimensions of the T-shaped two-dimensional electron gas could be varied continuously. Especially, the stub length of the transistor can be controlled in order to switch between constructive and destructive interference. An additional advantage of using gates instead of etching methods to define the geometry is the smooth form of the boundary potential which implies specular boundary scattering. At low temperatures the transport in the high mobility two-dimensional electron gas ͑2DEG͒ is ballistic. Thus weaklocalization effects and conductance fluctuations are suppressed, whereas the intended interference pattern is reproducible and nearly identical for different samples. We attribute the observed resistance oscillations to the change in transmissivity in the device when the geometry is altered. Other explanations are discussed as well but could be excluded by experiment.

This article presents numerical studies on multimode transport in a T-shaped quantum transistor geometry. Solving the time independent Schrödinger equation with adequate boundary conditions we model the current for up to six one-dimensional modes in the T structure. It is found, that independent of the number of modes periodic features dominate the conductivity as a function of gate voltage. Their origin is explained in terms of mode coupling in the stub region of the transistor for which the electron velocity in the waveguide is essential. The results are compared with experimental data.