  The elliptic flow $v_2$ and the ratio of the shear viscosity over the entropy
density, $\eta/s$, of gluon matter are calculated from the perturbative QCD
(pQCD) based parton cascade Boltzmann approach of multiparton scatterings. For
Au+Au collisions at $\sqrt{s}=200$A GeV the gluon plasma generates large $v_2$
values measured at the BNL Relativistic Heavy Ion Collider. Standard pQCD
yields $\eta/s\approx 0.08-0.15$ as small as the lower bound found from the
anti-de Sitter/conformal field theory conjecture.
