The current in a wire is due to the
motion of free electrons within the wire. These electrons are not bound to any particular atom
but are free to 'wander' through the body of the material.
The more free electrons
per unit volume the greater the current for a given voltage difference across the specimen.
So for specimens of different material of equal shape and size the one with the fewer free
electrons per unit volume will have the highest resistance. The ease with which these free
electrons can move through the metal also affects its resistance and this is determined by its
crystal structure.
Constantan is an alloy with a low free electron concentration and
so the resistance of a given size and shape of specimen will be relatively
large.
Note: Constantan is an alloy comprising copper (60%) and nickel (40%) and
has a resistance that changes very little with temperature.