I've been posting on that issue quite a while ago here and shortly saying there is a micro-diode theory of all conductors. It means that there are impurities of semi-conductive nature that reside inside the chrystalic structure with one-way conductivity i.e. micro-diodes. As to all semiconductive(one-way) elements(n or p) they have a break-in voltage when resistance of reverse connection becomes small or zero in ideal case.
These impurities if "connected" along the signal path have zero resistance and that's wy have almost no impact on transfering signal. The impurities that are oppisite the signal path have infinite resistance and the signal have to bypass them (like passing a double-parked vehicle on your line throu the parallel line) meaning that in this case the active resistance increases. But certainly the active resistance deviation isn't only an issue since such micro-diodes also act like non-linear elements i.e. time and freequency depended devices(even on as advertised pure conductors 99.99999%...) having some mutual and self reactances.
So, to generalize, when we apply signal to a wire we break-in some of micro-diodes depending on time and the amplitude of the input signal. Some inner micro-diodes may require larger amplitudes than we usually apply during audio playback and will theoretically never be broken-in. That's why a real break-in happens when you apply AC voltage(with proper load) onto the speaker wire or interconnects.
The micro-diode structure can only be visualized under the electronic microscope.
