PHOTOCONDUCTANCE OSCILLATIONS IN A 2-DIMENSIONAL QUANTUM POINT-CONTACT

We have calculated the photoconductance of a quantum point contact sub ject to a high-frequency electromagnetic held. We find very pronounced steplike oscillations of the photoconductance as a function of gate v oltage. The absorption of the electromagnetic held, which is polarized in the transverse direction, is due to transitions between different modes of the quantum point contact. A transition between a propagating and a nonpropagating mode results effectively in a backscattering pro cess, and gives a negative or positive contribution to the current, de pending on the gate voltage. When the number of propagating modes thro ugh the point contact exceeds a cutoff value, the photoconductance dis appears. The cutoff value depends on the frequency of the electromagne tic field. We also find that the oscillation amplitude increases with the number of propagating modes. As a result of the electron-photon in teraction, the total quantized conductance acquires an additional step structure.