  The recently discovered layered Kagome metals of composition AV3Sb5 (A = K,
Rb, Cs) exhibit a complex interplay among superconductivity, charge density
wave order, topologically non-trivial electronic band structure and geometrical
frustration. Here, we probe the electronic band structure underlying these
exotic correlated electronic states in CsV3Sb5 with quantum oscillation
measurements in pulsed fields up to 86 T. The high-field data reveal a sequence
of magnetic breakdown orbits that allows the construction of a model for the
folded Fermi surface of CsV3Sb5. The dominant features are large triangular
Fermi surface sheets that cover almost half of the folded Brillouin zone that
have not yet been detected in angle resolved photoemission spectroscopy
(ARPES). These sheets display pronounced nesting at the charge density wave
(CDW) vectors, which may stabilize the CDW state. The Berry phases of the
electron orbits have been deduced from Landau level fan diagrams near the
quantum limit without the need for extrapolations, thereby unambiguously
establishing the non-trivial topological character of several electron bands in
this Kagome lattice superconductor.
