Abstract When subjected to a high-field voltage sweep, the post-breakdown I– V characteristic of an ultrathin gate oxide exhibits current jumps, which are associated with the creation of area-distributed breakdown spots. The conductance of these spots is found to be of the order of the quantum conductance unit 2 e 2 /h, which indicates that the leakage paths connecting both electrodes have atomic-scale dimensions. In addition, the post-breakdown I–V characteristics often exhibit fluctuations, which include “anti-breakdown” events. We suggest that these latter events might be related to a rearrangement of defects within the breakdown paths. Electromigration effects are shown to be a reasonable assumption to explain the modification of breakdown paths at the atomic scale.