CONDUCTANCE OF QUANTUM POINT CONTACTS CALCULATED USING REALISTIC POTENTIALS

The quantized conductance of ballistic electrons in narrow constrictions requires the low scattering rates of a high mobility 2-dimensional electron gas(2DEG). Despite the spectacular mobilities of 2DEGs, quantization breaks down for constrictions longer than about 500 nm, whereas the mean free path in the ungated 2DEG may exceed 10 μm. We show that potential fluctuations from a random distribution of ionised donors modify the guiding potential from the gate in an unpredictable way. The fluctuations are responsible for the variability among nominally identical devices. For a short constriction (200 nm) the conductance can be well quantized. The accuracy of quantization in a longer constriction (600 nm) is reduced by scattering from the random potential of the donors. This occurs when the length scale of the constriction is greater than that of the fluctuations. The conductance characteristics depend strongly on the exact configuration of impurities. In some cases the random potential causes a resonance in the channel. © 1991.