What causes or reduces resistance in a material?

Resistance can be thought of as the amount of distruption to the flow of electrons. If we look at the equation for resitivity:R=kl/A,where k is the resistivity of the material (and is dependant on the material), l is the length of the material and A the area. We can see that as l increases in size so does R. While if we increase the size of A, R will get smaller. Your answer is shown in the algebra for you, the reason for this is that as the electrons flow through a material, the further they have to flow the more likely they are to be interruped. Wheras if the material has a larger area they can flow more freely with less chance of collision.