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Formation of narrow channels using split back-gates defined by in situ focused ion beam lithography

We have fabricated high electron mobility transistors incorporating split back-gates using the technique of in situ focused ion beam lithography and molecular beam epitaxial regrowth. We show that it is possible to form quasi-ballistic channels by biasing the split back-gate and have succeeded in defining wires of 400 nm width. Application of a small perpendicular magnetic field has allowed us to demonstrate that boundary scattering in these devices is mostly specular, as for conventional structures fabricated with Schottky gates.