Scanning electron micrographs of nanostructures in GaAs-AlGaAs heterostructures. (a, top) Narrow channel (width 75 nm), fabricated by means of the confinement scheme of Fig. 4c. The channel has side branches (at a 2-um separation) that serve as voltage probes. Taken from M. L. Roukes et al., Phys. Rev. Lett. 59, 3011 (1987). (b, bottom) Double quantum point contact device, based on the confinement scheme of Fig. 4b. The bar denotes a length of 1 μm. Taken from H. van Houten et al., Phys. Rev. B 39, 8556 (1989).