HOPG is a highly anisotropic material. I'm wondering if there is a straightforward direct method for measuring the in-plane electrical conductivity for square crystals of 1 mm thickness.
The most reliable is covering opposite edges with silver paint, to force the current to go throughout the sample only in a way strictly parallel to the surface. Then force a small current to flow, measure the corresponding voltage drop with a micro or nanovoltmeter, and measure accurately the distance between the silver paint borders and the thickness of the sample. Then apply Ohm's law. That simple.