We have investigated the response of the acoustoelectric current driven by asurface-acoustic wave through a quantum point contact in the closed-channelregime. Under proper conditions, the current develops plateaus at integermultiples of ef when the frequency f of the surface-acoustic wave or the gatevoltage Vg of the point contact is varied. A pronounced 1.1 MHz beat period ofthe current indicates that the interference of the surface-acoustic wave withreflected waves matters. This is supported by the results obtained after asecond independent beam of surface-acoustic wave was added, traveling inopposite direction. We have found that two sub-intervals can be distinguishedwithin the 1.1 MHz modulation period, where two different sets of plateausdominate the acoustoelectric-current versus gate-voltage characteristics. Insome cases, both types of quantized steps appeared simultaneously, though atdifferent current values, as if they were superposed on each other. Theirpresence could result from two independent quantization mechanisms for theacoustoelectric current. We point out that short potential barriers determiningthe properties of our nominally long constrictions could lead to an additionalquantization mechanism, independent from those described in the standard modelof 'moving quantum dots'.
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