  The continuing miniaturization of microelectronics raises the prospect of
nanometre-scale devices with mechanical and electrical properties that are
qualitatively different from those at larger dimensions. The investigation of
these properties, and particularly the increasing influence of quantum effects
on electron transport, has therefore attracted much interest. Quantum
properties of the conductance can be observed when `breaking' a metallic
contact: as two metal electrodes in contact with each other are slowly
retracted, the contact area undergoes structural rearrangements until it
consists in its final stages of only a few bridging atoms. Just before the
abrubt transition to tunneling occurs, the electrical conductance through a
monovalent metal contact is always close to a value of 2e^2/h, where e is the
charge on an electron and h is Plack's constant. This value corresponds to one
quantum unit of conductance, thus indicating that the `neck' of the contact
consists of a single atom. In contrast to previous observations of only
single-atom necks, here we describe the breaking of atomic-scale gold contacts,
which leads to the formation of gold chains one atom thick and at least four
atoms long. Once we start to pull out a chain, the conductance never exceeds
2e^2/h, confirming that it acts as a one-dimensional quantized nanowire. Given
their high stability and the ability to support ballistic electron transport,
these structures seem well suited for the investigation of atomic-scale
electronics.
