The signature of conductance quantization in metallic point contacts

An electrical current passing through a perfectly smooth narrow constriction is carried by a finite number of quantized modes (analogous to those in a waveguide), each of which contributes 2e sup 2 /h to the conductance *RF 1*. Conductance quantization has been observed in semiconductor devices containing a two-dimensional electron gas [2,3], where the width of the constriction is adjusted continuously by applying an electric field. A similar effect is expected to occur in three-dimensional metallic point contacts [4,5], and conductance steps of approximately 2e sup 2 /h have recently been observed [6-10]. But metallic point contacts do not change size continuously [6], and thus the conductance steps reported in these earlier experiments might be attributable to discrete rearrangements of the atomic structure of the contact, rather than true conductance quantization [11]. Here we use the fact that the degeneracy of the conduction modes of a three-dimensional point contact should result in a characteristic sequence of conductance values [4,5] (some integer multiples of 2e² /h are excluded) to distinguish the effects of conductance quantization from those of discrete variations in contact size in a break-junction experiment, confirming that conductance quantization does indeed occur.