I think public transit will be a problem - as I'm not aware that any take scooters. You can get a wheelchair on and off buses (but not Metro) and in almost all public buildings. But scooters are so much heavier/larger that they aren;t really an option. (I worked with someone who needed a scooter - and we had to get a small van with driver that held a scooter - but this may well be cost prohibitive for you.)

nytraveler, the RER stations that have lifts have lifts wide enough to accommodate a large scooter.

I was thinking the major issue in Paris is toilets. You can't just walk into a cafe to use their bathroom here (although seeing you have limited options they might make an exception) and many many cafes and restaurant have their toilet downstairs, sometimes down narrow staircases that are difficult even for the most mobile people. Luckily there are sanisettes all over Paris that are accessible and free. With the help of sites like sagetraveling.com and infomobi.com you should find other accessible toilets as well as ccessible restaurants and hotels. Make sure you have a list of accessible toilets before you go.

Once again, preparing your trip will require a little more homework than for most travelers but Paris is worth it, isn't it? :) In the 2008 thread I linked to yesterday several managed to enjoy Paris with a scooter so I would think that 4 years later it would be a little easier. Of course I don't know if your scooter is larger than theirs was. That is something to consider.