Planar nanoscopic contacts are observed to undergo early electrical breakdown. The authors show that the cause is high field emission capable of triggering electromigration. The phenomenon is well described by an empirical current-voltage law, well different from that usually found in nonflat field emitters; this is attributed to the particular geometry of the contacts. Although the mathematical form of the law is always the same, the intensity of breakdown current changes from sample to sample, ranging over several orders of magnitude; this is explained by the nanoscopic roughness of the emitting surfaces. They also show that the occurrence of breakdown may be dependent on the polarity of the applied voltage.