A 110 volt light bulb and a 1.5 volt mini light bulb are wired in series to a nine volt battery. What will happen when the electricity flows? This is a nice simple demonstration of how temperature can affect the resistance in a conductor. The result is called a positive temperature coefficient.
In metal conductors, there are already a large number of free electrons flowing through it, the vibrations of the atoms cause many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms bump around in the material, the more collisions there are and the friction causes the material to heat up, this then increases the resistance causing the decrease in current flow.