The breaking speed of the contact was incorrectly stated as 42 nm/s instead of 420 nm/s. This speed was used in Sec. IV B to estimate the displacement of the Al wire during ͗ Al ͘ and the tensile force accumulated within the contact. Their correct values are ϳ1 pm and ϳ10 pN, respectively. The corrected force is, however, still two orders of magnitudes smaller than a typical break force of metal single-atom contacts. Therefore, the error does not affect our argument that the stress accumulation is not the dominant breaking mechanism of Al and Au single-atom contacts under high biases.

Conductance and its bias dependence have been measured on Au and Ag breaking nanocontacts for biases up to 3.0 V at room temperature in ultrahigh vacuum. Under high-bias/high-current conditions, both Au and Ag contacts often show a characteristic conductance fluctuation when the conductance attains a certain critical value Gth. This critical conductance ranges from ∼ 10G0 to ∼ 50G0 (G0 ≡ 2e 2 /h is the quantum unit of conductance) and increases with the bias. When Gth is plotted against the contact current I, we obtained a linear I − Gth plot for Au contacts. Since Gth is in a semiclassical regime and hence should be proportional to the cross sectional area of the contact, the slope of the I − Gth plot represents a critical current density for the onset of the conductance fluctuation. These observations indicate that the conductance fluctuation is due to certain current-induced contact instability. When the current density exceeds the critical value, a contact becomes unstable and tends to be ruptured, leaving small chances of further necking deformation down to a single-atom contact.

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We measured the high-bias conductance of atom-sized Al contacts at room temperature for biases from 0.1 to 0.8 V, and studied the formation probability and the lifetime of Al single-atom contacts ͑SACs͒ under high biases. Analyses of conductance plateaus corresponding to Al SACs revealed that the formation probability of SACs decreases with increasing the bias and leads to the suppression of the first peak in the conductance histogram. The formation probability vanishes at a critical bias V bc ϳ0.8 V. On the other hand, the average plateau length, representing the average lifetime of SACs, decreases almost linearly with increasing the bias but remains finite at V bc . Similar results were also obtained for Au SACs but with a higher critical bias V bc ϳ2.4 V. We consider that the reduction of the formation probability of Al and Au SACs is due to a currentinduced contact instability, which takes place before the contact is reduced to a SAC. On the other hand, the mechanism of the observed linear bias dependence of the SAC plateau length is yet unclarified.

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