  We investigate the electronic eigenstates of graphene quantum dots of
realistic size (i.e., up to 80 nm diameter) in the presence of a perpendicular
magnetic field B. Numerical tight-binding calculations and Coulomb-blockade
measurements performed near the Dirac point exhibit the transition from the
linear density of states at B=0 to the Landau level regime at high fields.
Details of this transition sensitively depend on the underlying graphene
lattice structure, bulk defects, and localization effects at the edges. Key to
the understanding of the parametric evolution of the levels is the strength of
the chiral-symmetry breaking K-K' scattering. We show that the parametric
variation of the level variance provides a quantitative measure for this
scattering mechanism. We perform measurements of the parametric motion of
Coulomb blockade peaks as a function of magnetic field and find good agreement.
We thereby demonstrate that the magnetic-field dependence of graphene energy
levels may serve as a sensitive indicator for the properties of graphene
quantum dots and, in further consequence, for the validity of the
Dirac-picture.
