In a couple of weeks time, I'm off to the Loire by Eurostar + TGV, which means a cross Paris change. The Eurostar uses Gare du Nord, while the TGVs down to the Loire are at Gare Montparnasse.

One option is to take a taxi, but that looks to be around the [€30 mark](http://www.seat61.com/Paris-metro.htm#taxi). 

There is a direct metro line linking the two stations, M4, which would seem to be a great bet. Unfortunately, the metro station isn't really at Gare Montparnasse, but is instead [a 750m+ walk](http://www.seat61.com/Paris-metro.htm#Metro%20routes%20from) with lots of stairs. With a load of luggage, that's not looking much fun...

If I want to do the transfer by public transport, is that my only bet? Or is there another route (perhaps by bus?) that'll let me get from one station to the other by public transport, but without lots of walking?