We present measurements of current–voltage (I–V) curves on gold quantum point contacts (QPCs) with a conductance up to 4 G0 (G0=2e2/h is the conductance quantum) and voltages up to 2 V. The QPCs are formed between the gold tip of a scanning tunneling microscope and a Au(110) surface under clean ultra-high-vacuum conditions at room temperature. The I–V curves are found to be almost linear in contrast to previous reports. Tight-binding calculations of I–V curves for one- and two-atom contacts are in excellent agreement with our measurements. On the other hand, clearly nonlinear I–V curves are only observed when the sample has been cleaned in air.

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The conductance of a single-atom contact is sensitive to the coupling of this contact atom to the atoms in the leads. Notably for the transition metals this gives rise to a considerable spread in the observed conductance values. The mean conductance value and spread can be obtained from the first peak in conductance histograms recorded from a large set of contact-breaking cycles. In contrast to the monovalent metals, this mean value for Pt depends strongly on the applied voltage bias and other experimental conditions and values ranging from about 1 G0 to 2.5 G0 (G0 = 2e 2 /h) have been reported. We find that at low bias the first peak in the conductance histogram is centered around 1.5 G0. However, as the bias increases past 300 mV the peak shifts to 1.8 G0. Here we show that this bias dependence is due to a geometric effect where monatomic chains are replaced by single-atom contacts, where the former are destabilized by the electron current at high bias.

We present an experimental setup for measuring the electrical conductance through metallic quantum point contacts (QPCs) under constant or time-dependent bias voltage conditions. The response time of the setup is as short as 25 ns and typical bias voltages range from 10 mV to 2 V. A function generator is used as bias voltage supply. With this, voltage bursts with a frequency of up to 100 kHz can be applied to the QPCs, whereby current-to-voltage (I–V) curves can be acquired using a homebuilt, 30 MHz bandwidth I–V converter, and a 100 Msamples/s digital storage oscilloscope. Test experiments on resistors show that nonlinear contributions to the I–V curves are always less than 1% of the current for all applied voltages. From the slope of the I–V curves, the conductance can be determined with an accuracy better than 1%. The QPCs are formed between a single-crystal metal sample and the tip of a scanning tunneling microscope under clean ultrahigh vacuum conditions. We demonstrate how the setup can be used to capture the I–V curves of several metastable states in a Au QPC, as it breaks during a period of 200 μs at room temperature.

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