Materials which are classed as CONDUCTORS tend to INCREASE their resistivity with an increase in temperature. How does this happen?

Resistivity of the material of a conductor is inversely proportional to the average time between two successive collisions of free electrons (relaxation time). With increase in temperature, the collisions of free electrons become more frequent resulting in decrease in relaxation time. So, CONDUCTORS tend to INCREASE their resistivity with an increase in temperature.