  This study calculated the magnetoresistance (MR) in the Dirac electron
system, Dressellhaus-Kip-Kittel (DKK) model, and nodal-line semimetals based on
the semiclassical Boltzmann theory, with particular focus on the detailed
energy dispersion structure. The negative off-diagonal effective-mass was found
to induce negative transverse MR owing to the energy dispersion effect. The
impact of the off-diagonal mass was more prominent in case of a linear energy
dispersion. Further, Dirac electron systems could realize negative MR even if
the Fermi surface was perfectly spherical. The obtained negative MR in the DKK
model may explain the long-standing mystery in p-type Si.
