The authors have observed Aharonov-Bohm oscillations using a split gate GaAs-AlGaAs heterojunction FET of the type used to demonstrate the quantised, one-dimensional, ballistic resistance. The periodicity of the Aharonov-Bohm oscillations allowed extraction of the channel area and, hence, by using values of width derived from magnetic depopulation data, the channel length could be determined. It is found that the effective channel length increases as the area, and width, decreases in agreement with a simple model based on the depletion approximation.