Small structures in high‐mobility semiconductor heterojunctions are expected to show a variety of quantum mechanical phenomena associated with electron interference. At low temperatures in the high‐mobility two‐dimensional electron gas at a GaAs/AlGaAs heterojunction, both the elastic and inelastic electron scattering lengths are several microns, making it possible to fabricate complete devices contained within these dimensions. In ring shaped devices we have observed periodic magnetoconductance oscillations (Aharonov–Bohm effect) caused by electron interference. The magnitude of this effect is as much as 10% of the total conductance, much larger than seen in previously studied metal systems. High‐resolution electron (e)‐beam lithography and reactive ion etching were used to fabricate rings of 1, 2, and 2.5 μm mean diameters with linewidths of 0.5 μm. The electrical channel width is smaller than the physical channel width because of edge depletion. Samples with conducting channels varying from about 0.6 to 0.2 μm have been made. The magnetoconductance of these rings oscillate with a period ofhc/e(±2%) based on the mean diameter, as expected for a single electron interference effect. The Hall resistance of these samples show strong nonlocal effects, demonstrating that the channel acts as a phase preserving electron waveguide. These devices are an excellent test system for studying electron transport in the extreme quantum limit, and exploring the consequences of the limits of microfabrication.