  Due to the chiral nature of electrons in a monolayer of graphite (graphene)
one can expect weak antilocalisation and a positive weak-field
magnetoresistance in it. However, trigonal warping (which breaks p/-p symmetry
of the Fermi line in each valley) suppresses antilocalisation, while
inter-valley scattering due to atomically sharp scatterers in a realistic
graphene sheet or by edges in a narrow wire tends to restore conventional
negative magnetoresistance. We show this by evaluating the dependence of the
magnetoresistance of graphene on relaxation rates associated with various
possible ways of breaking a 'hidden' valley symmetry of the system.
