We have directly observed individual hopping events in the flow of curent along a single chain of localized states. These events modulate the conductance of a ballistic constriction formed in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As split-gate transistor, resulting in a time irreversible random telegraph signal. The hopping chain, carrying approximately one electron per second, is situated between the gate and the two-dimensional electron gas, and can be altered by varying the gate voltage.

• Received 18 May 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.502