The easiest way is to sketch a rectangle (depicting the cross section of the material) and sketch a line which will be the center line of the coil. Finish sketch and pick the COIL command. Under coil size, pick spiral and input the parameters you need (number of revolutions, and dimension that each revolution increases in size). NOTE: the coil command is very math intensive and will suck up the CPU time and memory. BE patient.
If the part turns out to be too large to deal with in an assembly, you could also make a solid cylinder and attach a texture to each end to make it look like a coil (just like the thread texture on bolts)