We present measurements of the electrical conductance through breaking point contacts in commercial and home-built electromechanical relays under ambient conditions. Inspired by an experiment published by Costa-Krämer et al. [Surf. Sci. 342, L1144 (1995)], we constructed an automated setup, which makes it possible to record the conductance in a breaking contact 25 000 times during 24 h. Quantization of the conductance in units of $G_0{=2e}^2/h$ is shown to be intimately correlated with the choice of contact material and experimental conditions. This and previous studies clearly demonstrate that Au is a benchmark system, which exhibits quantized conductance independent of the conditions. Peaks in the Au histogram are shown to be asymmetric, which is interpreted as another indicator of conductance quantization. For the more reactive metals Ag and Cu, quantization is also observed, but not as clearly as in Au. In contrast, we find no evidence of quantization in a number of transition metals. It is tentatively suggested that the presence and/or absence of quantization is correlated with impurities in the nanowires. Other deviations from perfect quantization are discussed. It is revealed how differential nonlinearity of analog-to-digital converters introduces significant noise in the corresponding conductance histograms unless a proper compensation is made.

• Received 31 January 1997

DOI:https://doi.org/10.1103/PhysRevB.56.2208