There are brazing alloys of 50% silver and 50% copper that would do nicely in terms of heat conductivity. I cannot find any tech sheets on their specific heat conductivity, but alloys combine the features of metals. Both copper and silver are great conductors of heat, so the alloy of the two should perform at least on par with copper.
This is something that might be used:
http://www.lucasmilhaupt.com/assets/...eets/BR559.pdf