  We present the magnetoresistance (MR) of highly doped monolayer graphene
layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport
studies are performed on a large temperature range, from $T$ = 1.7 K up to room
temperature. The MR exhibits a maximum in the temperature range $120-240$ K.
The maximum is observed at intermediate magnetic fields ($B=2-6$ T), in between
the weak localization and the Shubnikov-de Haas regimes. It results from the
competition of two mechanisms. First, the low field magnetoresistance increases
continuously with $T$ and has a purely classical origin. This positive MR is
induced by thermal averaging and finds its physical origin in the energy
dependence of the mobility around the Fermi energy. Second, the high field
negative MR originates from the electron-electron interaction (EEI). The
transition from the diffusive to the ballistic regime is observed. The
amplitude of the EEI correction points towards the coexistence of both long and
short range disorder in these samples.
