  We present measurements of mesoscopic resistance fluctuations in cobalt
nanoparticles and study how the fluctuations with bias voltage, bias
fingerprints, respond to magnetization reversal processes. Bias fingerprints
rearrange when domains are nucleated or annihilated. The domain-wall causes an
electron wavefunction phase-shift of $\approx 5\pi$. The phase-shift is not
caused by the Aharonov-Bohm effect; we explain how it arises from the
mistracking effect, where electron spins lag in orientation with respect to the
moments inside the domain-wall. Dephasing time in Co at $0.03K$ is short,
$\tau_\phi\sim ps$, which we attribute to the strong magnetocrystalline
anisotropy.
