Electrons are constantly colliding with atoms in the metal's lattice and with other electrons.The frequency of collisions is determined by the temperature as this affects how much the ions vibrate.The higher the temperature the more the atoms vibrate and therefore the greater number of collisions.This reduces the drift velocity of the electrons.
A greater current will also increase the frequency of collisions, increasing the temperature and as a result increasing the resistance.
A greater current will also increase the frequency of collisions, increasing the temperature and as a result increasing the resistance.