The concept for Break -in for headphones is pure horse crap, and so it is for cables.
In terms of scientific terms - entropy, the conductivity of the metal experiences creep with time, that is the conductive capacity decreases with time, and so it would imply a reverse phenomenon - i.e. degrading sound quality along with use and time.
The case of break in is only somewhat true in case of Speakers, since the speaker drivers need a bit of time to loosen up, and they draw a fair amount of power for the vibrations to take place.