The fabrication of a nanoscale gated quantum wire in a GaAs modulation doped field effect transistor substrate is described. Both the wire conduction channel, with a 110 nm physical width, and the gates were patterned into the two‐dimensional electron gas of the substrate. This scheme produced in‐plane gated devices with 110 nm gate lengths and 75 nm separations between the active areas. Electron‐beam lithography was used to define masks for a subsequent flood exposure step with low energy argon ions (150 eV). This ion exposure technique produced very high gate‐to‐wire isolation, typically greater than 1014Ω at 4.2 K. The in‐plane design employed here drastically reduces gate capacitance compared with metal top‐gate designs, and promises ultrafast switching times. These devices showed no short channel punch‐through effects, exhibited low gate leakage, and had sufficient gain to permit integration of several such devices into more complex circuits such as logic gates.