Conduction electron spin resonance and relaxation in the superconducting state.

We report the first observation of conduction electron spin resonance (CESR) in both the normal and superconducting states of pure niobium. Surprisingly, the resonance displays a g-value of 1.84 +- 0.01 in both states. We show that this narrowing is a consequence of the coherence factors which, in BCS theory, modify all scattering matrix elements.