Fast and accurate current-voltage curves of metallic quantum point contacts

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 mu s at room temperature. (C) 2000 American Institute of Physics. [S0034-6748(00)01504-5].