Electromigration Limits

A localized M4 power rail carries a continuous, unidirectional DC current of 10mA. The foundry design manual sets the Electromigration (EM) safe current density limit for M4 at 2mA per micrometer (µm) of metal width. If the physical router assigns this net a wire width of 2µm, what is the most likely physical failure mechanism over a multi-year operation span?