What effect does heating have on metals (to be specific, on the electrical conductivity) ? And why does it have that effect?
What effect [does] heating have on [the electrical conductivity of] metals?
Conductivity drops as temperature rises
Note, this graph plots resistivity not conductivity, vs temperature.
Temperature obviously affects the thermal velocity of free electrons and this affects the rate of collisions. This affects drift velocity and hence current.
Obviously, sufficient heating to cause a change of phase would likely have a more dramatic effect on electrical conductivity.