I'm not an EE, so I have no clue what happens during "break in", but my common sense tells me that, as Dr. Greenman notes, it doesn't have anything to do with a restructuring of the conductive metal. To me, it seems more reasonable to suppose that the dielectric undergoes some small changes. All I really know is the evidence of my own fallible senses, and they tell me that I often notice a change following a cable break in period.