Imaging coherent electron flow from a quantum point contact

Scanning a charged tip above the two-dimensional electron gas inside a gall ium arsenide/aluminum gallium arsenide nanostructure allows the coherent el ectron flow from the lowest quantized modes of a quantum point contact at l iquid helium temperatures to be imaged. As the width of the quantum point c ontact is increased, its electrical conductance increases in quantized step s of 2 e(2)/h, where e is the electron charge and h is Planck's constant. T he angular dependence of the electron flow on each step agrees with theory, and fringes separated by half the electron wavelength are observed. Placin g the tip so that it interrupts the flow from particular modes of the quant um point contact causes a reduction in the conductance of those particular conduction channels below 2 e(2)/h without affecting other channels.