We fabricated quantum wires of different geometries in Ga[Al]As heterostructures by local oxidation of the semiconductor surface with an atomic force microscope. By magnetotransport measurements at low temperatures on these wires the electronic width is determined and compared to the geometrical width. An extremely small lateral depletion length of the order of 15 nm and a high specularity of the scattering at the confining walls is found. Furthermore, we demonstrate experimentally that these quantum wires can be tuned by a combination of in-plane gates and top gates.