Ackshually, as I dug into self-help approaches and business methodologies over the years, it's become very clear that it's important to learn to say “no” with abandon. Pretty much begin by assuming that refusal is the correct answer, and then figure out if there's reason to believe otherwise. In the OP case, I don't have any idea what the other party wants to talk about and therefore whether I can talk about that, so no. They should write to me what they want to ask about, then I'll see if the whole thing might just be a text reply, or perhaps I actually need to do more research. Even if we do schedule a meeting, the topic will bounce around in my head a bit in the meantime, and I will be better prepared.

Jim Camp's book ‘Start with No’ is a good introduction to this for some people, though I remember that it seemed somewhat too cheesy and obvious to me.