Heat Transfer Media

Reliable sources say a stream of helium gas is a very good coolant. Why is that? As a monatomic gas it has no vibrational modes, so it should have
a low specific heat. But a high specific heat is not necessary for a good coolant. It's the heat transfer rate that matters, not the amount of heat-in-transit. I'm guessing the low molecular weight contributes by increasing particle speed for a given amount of energy, hence increasing heat transfer rate.
Along the same lines, why are beryllium oxide and diamond excellent heat conductors. I'm guessing in the case of BeO, it's the same as helium except the atoms are constrained within the ceramic lattice. It's like solid helium.
Diamond is more of a puzzle to me. I'd guess it's a really tight lattice, so it's not as lossy a lattice as a looser one like water ice. It rapidly and efficiently transmits heat like a good bell metal rapidly and efficiently transmits acoustic waves.
Am I way off base here, or are these reasonable layman's models for the heat-transfer properties of these materials? If wrong in certain aspects, can they be modified or fixed to bring them more in line with current understanding?
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