We have investigated the electronic properties of the confining potentials obtained by oxidizing the surface of Ga[Al]As heterostructures with an atomic force microscope. From magnetotransport measurements on quantum wires at liquid-helium temperatures we find an extremely small lateral depletion length of (15±5) nm as well as a specularity of the boundary scattering above 95%. Furthermore, we demonstrate that this patterning technique for in-plane gates can be combined with top gate electrodes. Sufficiently thin top gates consisting of a suitable material can be patterned as well with an atomic force microscope, and the top gate structures can be aligned with respect to the in-plane gates.