  By using Fourier-transform scanning tunneling spectroscopy we measure the
interference patterns produced by the impurity scattering of confined Dirac
quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the
experimental results with tight-binding calculations of realistic model flakes,
we show that the characteristic features observed in the Fourier-transformed
local density of states are related to scattering between different transverse
modes (sub-bands) of a graphene nanoflake and allow direct insight into the
electronic spectrum of graphene. We also observe a strong reduction of
quasiparticle lifetime which is attributed to the interaction with the
underlying substrate. In addition, we show that the distribution of the onsite
energies at flower defects leads to an effectively broken pseudospin selection
rule, where intravalley back-scattering is allowed.
