Other techniques have been used as well to fabricate narrow electron gas channels. We mention selective-area ion implantation using focused ion beams,81 masked ion beam exposure, strain-induced confinement,83 lateral p-n junctions, gates in the plane of the 2DEG, and selective epitaxial growth. For more detailed and complete accounts of nanostructure fabrication techniques, we refer to Refs.9 and13,14,15