Does the conductivity of a wire in a vacuum decrease over time? Does the conductivity of a wire in a vacuum decrease over time, say over the period of years or decades? In other words: Does current degrade a wire, making it less conductive? If so, by how much, and why does this occur? Does it have something to do with the 2nd Law of Thermodynamics?
(I'm not looking interested in mechanical effects of currents on wire degradation, but thermodynamic effects.)
 A: The line itself does not change much over years.  What changes and therefore needs maintenance on power transmission lines is insulators, connectors and spacers.  Insulators get dirty or simply break, connectors work loose due to thermal expansion and contraction, mechanical stresses and oxidation, and spacers can be damaged by wear due to these same physical forces.
Edit due to change in question:
The original question asked about degradation of power transmission lines over time.  The edited version asks a completely different question about conductivity over time in a vacuum.  This edit adds a an answer to that new version of the question:
In short, physically, yes, there is an effect on the conductor over time.  It's called electromigration and is due to the migration of ions within the conductor.  This effect is noticeable at micron-scale and below, (e.g. within integrated circuits), however with larger conductors electromigration effects are largely irrelevant within meaningful human timespans.
A: Some corrosion always takes place (pure gold is not used for transmission lines, AFAIK:-) ), so the conductivity decreases with time, although for some materials this effect can be very small (http://books.google.com/books?id=IWn9uuISVIoC&pg=PA109&lpg=PA109&dq=transmission+line+corrosion+conductivity&source=bl&ots=Qt5Z3a9Irs&sig=0hoDGt_x9F-UtTUqk8eDReqfEbw&hl=en&sa=X&ei=xRlsVIOKOoqhNsiPgqAE&ved=0CEQQ6AEwCTgU#v=onepage&q=transmission%20line%20corrosion%20conductivity&f=false )
