THE SIGNATURE OF CONDUCTANCE QUANTIZATION IN METALLIC POINT CONTACTS

An electrical current passing through a perfectly smooth narrow constr iction is carried by a finite number of quantized modes (analogous to those in a waveguide), each of which contributes 2e(2)/h to the conduc tance(1). Conductance quantization has been observed in semiconductor devices containing a two-dimensional electron gas(2,3), where the widt h of the constriction is adjusted continuously by applying an electric field. A similar effect is expected to occur in three-dimensional met allic point contacts(4,5), and conductance steps of approximately 2e(2 )/h have recently been observed(6-10). But metallic point contacts do not change size continuously(6), and thus the conductance steps report ed in these earlier experiments might be attributable to discrete rear rangements of the atomic structure of the contact, rather than true co nductance quantization(11). Here we use the fact that the degeneracy o f the conduction modes of a three-dimensional point contact should res ult in a characteristic sequence of conductance values(4,5) (some inte ger multiples of 2e(2)/h are excluded) to distinguish the effects of c onductance quantization from those of discrete variations in contact s ize in a break-junction experiment, confirming that conductance quanti zation does indeed occur.