reference source for copper conductivity vs. temperature?
Data from NIST (table 2)

At low temperatures the resistivity of copper approaches a "residual" resistivity. However, this isn't really a concern until below ~100K.

At higher temperatures the resistivity is best approximated with a linear increase vs. temperature (as you've already found). I plotted the resitivity vs. temperature, and I wouldn't be too worried about non-linearity too much until ~800K.