Flying from NY to Paris to Nantes

Are you purchasing the flights or one itinerary/ticket? Customs and immigration/passport control are two different things. Whether you have two tickets or not, you will go through immigration/passport control at CDG. If you have two tickets customs will also be at CDG. As to your luggage, that is a question for your airline. Generally, if you purchase a connecting flight from an airline, your luggage will be checked through to your final destination but that is not the case 100% of the time and it is not the case if you purchase two separate tickets. Also, theoretically, airlines do not sell ticket with impossible connections but despite that I still worry about tight connections, especially in airports that I am not familiar with or that have a reputation for delays. If you have two separate tickets, then you absolutely have to be mindful of the connection time because if you miss your flight to Nantes, neither airline will have any obligation to assist you with getting there.

This brings up something that has puzzled me, maybe because I've never thought of it before when I've traveled. If, in this example, you have a connecting flight (one ticket) in Paris with checked baggage sent through to Nantes, when you get off in Nantes and pick up your luggage, you go through customs. How do the authorities in Nantes separate out all those Paris-Nantes travelers who took a domestic flight with checked baggage? Or doesn't it matter because Customs in France is just a "Nothing to Declare" door you walk through?

Robert,
All luggage labels issued in EU airports have a green line down the sides. Example here: http://ec.europa.eu/taxation_customs/resources/images/etiquette_verte.gif
That way customs can tell at a glance which bags are subject to customs and which not. The same would apply if you were catching a flight to another EU destination, for example Rome or London. The EU is a Customs Union, there is no customs between EU countries, and no customs limits or charges. Travelling into the EU from outside, customs limits and charges apply.

Full details of the scheme and rules are here: http://ec.europa.eu/taxation_customs/customs/customs_controls/baggage_controls/index_en.htm

Thanks very much Chris.