The signature of conductance quantization in metallic point contacts

Abstract

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 2e²/ h to the conductance¹. 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 2e²/ h have recently been observed^6–10. But metallic point contacts do not change size continuously⁶, 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 quantization¹¹. 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.