  The resistivity and magnetoresistance in the normal state for bulk and
thin-film MgB2 with different nominal compositions have been studied
systematically. These samples show different temperature dependences of normal
state resistivity and residual resistance ratios although their superconducting
transition temperatures are nearly the same, except for the thin-film sample.
The correlation between the residual resistance ratio (RRR) and the power law
dependence of the low temperature resistivity, rho vs. T^c, indicates that the
electron-phonon interaction is important. It is found that the
magnetoresistance (MR) in the normal state scales well with the RRR, a0(MR)
proportional to (RRR)^2.2 +/- 0.1 at 50 K. This accounts for the large
difference in magnetoresistance reported by various groups, due to different
defect scatterings in the samples.
