  We fabricated and measured antidot lattices in single layer graphene with
lattice periods down to 90 nm. In large-period lattices, a well-defined quantum
Hall effect is observed. Going to smaller antidot spacings the quantum Hall
effect gradually disappears, following a geometric size effect. Lattices with
narrow constrictions between the antidots behave as networks of nanoribbons,
showing a high-resistance state and a transport gap of a few mV around the
Dirac point. We observe pronounced weak localization in the magnetoresistance,
indicating strong intervalley scattering at the antidot edges. The area of
phase-coherent paths is bounded by the unit cell size at low temperatures, so
each unit cell of the lattice acts as a ballistic cavity.
