  The wave nature of electrons in low-dimensional structures manifests itself
in conventional electrical measurements as a quantum correction to the
classical conductance. This correction comes from the interference of scattered
electrons which results in electron localisation and therefore a decrease of
the conductance. In graphene, where the charge carriers are chiral and have an
additional (Berry) phase of \pi, the quantum interference is expected to lead
to anti-localisation: an increase of the conductance accompanied by negative
magnetoconductance (a decrease of conductance in magnetic field). Here we
observe such negative magnetoconductance which is a direct consequence of the
chirality of electrons in graphene. We show that graphene is a unique
two-dimensional material in that, depending on experimental conditions, it can
demonstrate both localisation and anti-localisation effects. We also show that
quantum interference in graphene can survive at unusually high temperatures, up
to T~200 K.
