Molecules are made up of numerous subatomic particles, so don't they constantly interact? How can we achieve quantum coherence? ($\rm C_{60}$) Since $\rm C_{60}$ is a molecule made up of numerous subatomic particles, no matter how separated from the environment, the interaction between $\rm C_{60}$'s subatomic particles inevitably causes quantum decoherence, right?  But how did quantum tunneling happen?
 A: Decoherence is a relative term. If the many particle system is in a pure state (which it will be in the absence of an environment) you can do one of two things. You can perform experiments on the system as a whole and see all predictions of quantum mechanics reproduced. Or you can limit yourself to only one carbon atom. The other 59 would then become "the environment". Since interactions with them are significant, the state being measured would appear to be classical even though it's part of the exact same many particle quantum pure state as before. The latter is entangled so tracing out part of the Hilbert space (a practical choice by the experimenter, not part of the dynamics) is what makes it mixed.
This is what decoherence is. A gradual increase in the power and sensitivity your experiment needs in order to still see quantum behaviour. It is not some spontaneous removal of quantum behvaiour which happens in any system that "knows" it's not free. According to every experiment so far, quantum mechanics is unitary and therefore does not contain the seeds of its own destruction.
