why"for small or stiff systems the dynamics is not ergodic"?

jiadong

Why"for small or stiff systems the dynamics is not ergodic"? [1]

For small system, the fluctuation of total energy is also very small. Then it can not overcome the potential barrier and can not reach other microstates which are possible under the same macrostate. Does this answer right? Could anyone explain the nonergodicity for the stiff systems?

[1] Martyna etc, J chem. Phys. 97(4)2635 (1992).