  Non-linearities in the harmonic spectra of hadron-nucleus and nucleus-nucleus
collisions provide evidence for the dynamical response to azimuthal spatial
eccentricities. Here, we demonstrate within the framework of transport theory
that even the mildest interaction correction to a picture of free-streaming
particle distributions, namely the inclusion of one perturbatively weak
interaction ("one-hit dynamics"), will generically give rise to all observed
linear and non-linear structures. We further argue that transport theory
naturally accounts within the range of its validity for realistic signal sizes
of the linear and non-linear response coefficients observed in azimuthal
momentum anisotropies with a large mean free path of the order of the system
size in peripheral ($\sim 50 \%$ centrality) PbPb or central pPb collisions.
The shear viscosity to entropy density ratio $\eta/s$ of such a transport
theory is approximately an order of magnitude larger than that of an almost
perfect fluid. The phenomenological success of transport simulations thus
challenges the perfect fluid paradigm of ultra-relativistic nucleus-nucleus and
hadron-nucleus collisions.
