Wearing a Malbush (garment) in the Reshut Harabim (public domain) is not considerd carrying. The question is how to define a garment. According to the Halacha, "a Malbush for one is a Malbush for all." That is, any garment that is normally worn by a specific group of people will be considered a garment, even if worn by someone else, who would ordinarily not wear such clothing. For example, if shepherds wear sackcloth as their usual garb, then anyone else can also wear it on Shabbat. A contemporary application would be wearing Hasidic garb by a non-Hassid or vice-versa.
There is also a question as to whether it is permitted to wear oversized or loose fitting clothing in the public domain. The Halacha clearly established that loose fitting Jewelry is problematic, as it may fall off and be carried. The fact that Rambam explicitly prohibits only the loose-fitting jewelry, can be understood as a leniency with regard to the clothing. In fact, the Shulhan Aruch brings a "Yesh Omrim" (minority opinion) that it is prohibited to wear a loose hat on Shabbat. Again, it can be inferred that the majority opinion permits such a hat as well as other loose fitting clothing.
One of the defining characteristics of a garment is that it is meant to protect, cover or warm the body of its wearer. However, "Asuleh Tinuf", a garment or accessory designed to protect another garment being worn is not Halachically defined as a garment and may not be worn. Therefore, the Shulchan Aruch rules that a man may not wear a protective "Kis" (pocket) to protect his garments from being soiled by bodily emissions. Based on this principle, the question arises as to whether it is permissible to wear a raincoat on Shabbat? While one may argue that in the winter, a raincoat also serves to provide warmth, nevertheless, in the summer, its sole purpose it to protect ones clothes from rain. The Shulchan Aruch rules that as long as the protective gear is worn in the normal manner that people wear a garment, it is permitted. The protective pocket is prohibited because that is not a normal type of clothing. Therefore, it is permitted to wear a raincoat in the public domain, even when it is not raining, and there is no concern that one will remove the raincoat and carry it.
SUMMARY
A piece of clothing ordinarily worn by one type of person, may be worn in the public domain by someone else who does not usually wear such a garment.
It is permitted to wear loose or oversized clothing in the public domain.
It is permitted to wear a raincoat in the public domain even when it is not raining.
