Because materials interfaces create weird effects. You would use metals as your leads and create Josephson-junctions-like interfaces. Basically the abrupt cha

Because materials interfaces create weird effects. You would use metals as your leads and create Josephson-junctions-like interfaces. Basically the abrupt change in electron mobility across materials can cause knock on effects that dominate what you are trying to measure. Interfacial engineering is one term in materials science that implements best practices for dealing with such challenges.