DURING the sweltering heat wave earlier this month, it seemed too hot to wear much, carry much or do much of anything at all. Every time I left the house, I tried to figure out where to stuff my bulky wallet. I always had room for my iPhone, even if it meant carrying it in my hand. But the wallet was one thing too many.

A truly mobile wallet — one that would let you easily pay for restaurant meals, subway rides or beers at a bar with a quick wave of your cellphone — has long been described as imminent. But it remains elusive. Some innovations have begun to bridge the gap, but most have been a disappointment or have not yet worked well enough for mainstream adoption.

In 2012, Square, which makes a credit card reader that can be plugged into an iPhone or iPad, worked on a credit-cardless system that let people pay for goods without ever pulling out their wallets or phones. When Square users walk into a store in its network, a Square-enabled register shows pictures of their faces, which are used as authentication for payment. But the app can be awkward to use.

Last summer, Apple introduced Passbook, a digital system for storing boarding passes, movie tickets, loyalty cards and gift cards on the iPhone. But it doesn’t do much beyond that, at least not yet. Google worked with major credit card companies and banks to create its Wallet app, which lets people pay for items at some stores by waving their phones but is available only for Android devices. Visa offers two digital wallets, payWave and V.me, but I’ve never seen anyone use them or signed up myself. And the major mobile carriers in the United States banded together to form Isis, a mobile payments network, which has yet to roll out nationally.