We study the electrical conductance of gold nanoconstrictions by controlling the electrochemical potential. At positive potentials, the conductance is quantized near integer multiples of $G_0({2e}^2/h)$ as shown by well-defined peaks in the conductance histogram. Below a certain potential, however, additional peaks near ${0.5G}_0$ and ${1.5G}_0$ appear in the histogram. The fractional conductance steps are as stable and well defined as the integer steps. The experimental data are discussed in terms of electrochemical-potential-induced defect scattering and Fermi energy shift, but a complete theory of the phenomenon is yet to be developed.

• Received 13 October 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.5196