that approaches two times the conductance quantum, . The factor of two arises because carbon nanotubes have two spatial channels. The structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n − m is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting. See also Conductance quantum Quantum point contact Quantum well Quantum dot References Nanowire Quantum electronics Semiconductor structures Mesoscopic physics