Early theoretical work on single-walled carbon nanotubes(1-3) predicted that a special achiral subset of these structures known as armchair nanotubes(3) should be metallic. Tans et al.(4) have recently confirmed these predictions experimentally and also showed directly that coherent electron transport can be maintained through these nanowires up to distances of at least 140 nm. But single-walled armchair nanotubes are one-dimensional conductors with only two open conduction channels (energy subbands in a laterally confined system that cross the Fermi level)(1-3). Hence, with increasing length, their conduction electrons ultimately become localize(5) owing to residual disorder in the tube which is inevitably produced by interactions between the tube and its environment. We present here calculations which show, however, that unlike normal metallic wires, conduction electrons in armchair nanotubes experience an effective disorder averaged over the tube＇s circumference, leading to electron mean free paths that increase with nanotube diameter. This increase should result in exceptional ballistic transport properties and localization lengths of 10 mu m or more for tubes with the diameters that are typically produced experimentally(6).